Well borehole systems that measure properties of earth formation, as a function of depth along the borehole, are typically referred to as borehole “logging” systems. Although formation lithology, formation mechanical properties and borehole parameters are of interest in hydrocarbon production, fluid properties of formations penetrated by the borehole are typically primary parameters of interest. NMR logging systems respond primarily to fluid parameters of the formation. Other types of hydrocarbon borehole logging systems typically require independent determinations of other formation properties in order to obtain the desired formation fluid parameters.
NMR logging systems can be used to determine free fluid volume (FFV) contained in formation pore space, irreducible water saturation or bound fluid volume (BFV), formation porosity, formation permeability, and fluid type (e.g. oil or water). In the type of NMR logging system that this patent addresses proton spin alignment within the formation is induced magnetically by a strong permanent magnet. An RF coil is then pulsed perturb this alignment. These pulses create echos from the protons in the formation which are received by the same or another RF coil. The measured echos are used to determine one or both of two principle relaxation times T1 and T2. These relaxation times are in turn used to determine formation and formation fluid properties of interest.
NMR borehole instrumentation, typically referred to as a borehole logging “tool”, can be conveyed along a borehole by means of a wireline, a production tubing string, or a borehole drill string. Tools conveyed by a drill string allow NMR measurements to be made while the borehole is being drilled. This technique, commonly known as logging-while-drilling (LWD) or measurement-while-drilling (MWD). Tools conveyed by a wireline or a string of production tubing allow NMR measurements to be made after the borehole is drilled. Each technique results in a variety of operational, economic and technical advantages and disadvantages, depending upon well location, well configuration, well conditions, apriori knowledge of the area being drilled and produced, and the like.
A typical NMR logging system comprises a borehole instrument that generates a static magnetic field H0 in the borehole, with a direction of magnetization running primarily perpendicular to the axis of the borehole. The instrument employs a coil to generate an exciting radio frequency (RF) field H1 in a direction essentially perpendicular to both the borehole axis and the static magnetic field H0. The RF field is pulsed at a frequency, duration, and power level to cause the protons in the formation to precess about the H0 field and to cause a series of echos from the formation. These echos are received by the same or another RF coil and are translated by the computer circuitry to measurements of relaxation times T1 or T2. See for example Carr et al., “Effects of Diffusion on Free Precision in Nuclear Magnetic Resonance Experiments,” Physical Review, vol. 94, No. 3 (May 1, 1954), pp. 630–638. These relaxation times are then converted to formation and fluid properties. See for example Timur, A., “Pulsed Nuclear Magnetic Resonance Studies of Porosity, Movable Fluid and Permeability of Sandstones”, J. of Petrol Tech., June 1969.
The static magnetic field H0 is preferably generated with a permanent magnet. The permanent magnet is either a conductive magnet made from rare earth elements such as samarium cobalt, or by a non-conductive magnet such as a hard ferrite magnet. Conductive magnets are stronger than hard non-conductive magnets of comparable size. Weaker non-conductive magnets produce a corresponding weaker H0 field which translates to a lower signal to noise NMR measurement. Conductive properties of the stronger conductive magnet inhibit RF antenna gain by supporting eddy currents in the conductive magnetic body.
Instruments utilizing these basic concepts are disclosed in WO 02/057809 A1, and in U.S. Pat. Nos. 4,710,713, 6,268,726 B1, and 6,362,619 B2.
U.S. Pat. No. 4,710,713, which is incorporated herein by reference, discloses a wireline logging tool, which uses a cylindrical, non-conductive, permanent hard ferrite magnet to produce the static magnetic field H0. A coil is disposed around the magnet whose turns are parallel to the magnetic field. The coil produces a RF field H1 that is essentially perpendicular to the static field H0. It is taught that it is essential for the permanent magnet material to be non-conductive. The RF field H1 produced by the coil is therefore not obstructed by a conductive structure that would produce eddy currents, which would result in an unwanted reduction in the RF field strength. The advantages in the H1 field are obtained at the expense using a weaker non-conductive magnet, which produces a corresponding weaker H0 field.
U.S. Pat. Nos. 6,268,726 B1, and 6,362,619 B2 disclose MWD logging tools which use a conductive permanent magnet to generate a static H0 field, and use ferrite to reduce the effects on an RF antenna on a conductive drilling collar in which the tool is housed. Ferrite is not used to reduce adverse effects of the RF antenna on the conductive permanent magnet. These references also disclose auxiliary coils in addition to the RF antenna coil. Auxiliary coils are used as a field monitor in a feedback calibration circuit.
WO 02/057809 A1 discloses a system employing a permanent conductive magnet to generate a static H0 field, and an RF coil to generate an essentially perpendicular H1 field. An additional RF coil is disposed in the magnet-coil assembly to cancel the RF field in the vicinity of the conductive magnet body thereby minimizing eddy currents within the body of the magnetic. The additional coil, however, effectively reduces the magnetic moment of the H1 coil by the area of the conductive area cross section, again translating into a lower RF field in the formation penetrated by the well borehole. The lower RF field in the formation results, in turn, in a lower signal to noise NMR signal measurement from the system with a corresponding reduction in precision in measures of parameters of interest.